Semiconductor light emitting devices such as laser diodes, light emitting diodes and superluminescent diodes (SLD) are extensively used in many applications. Some of these applications require emitters that combine high brightness with a very broad optical spectrum, preferably as broad as tens or even hundredth nanometers (nm); these applications include the optical coherence tomography (OCT), low-coherence spectroscopy, and broad band optical amplification in an optical amplifier or tunable laser. For such application the superluminescent diodes, which provide optical amplification in the absence of lasing and are therefore characterized by a relatively high emission intensity in combination with a relatively broad optical spectrum, are typically used.
However, the emission bandwidth of a conventional SLD is limited by its material properties, which define the spectral shape and width of the SLD emission for a device of a given length. The emission spectrum of a typical SLD has a full width half maximum (FWHM) bandwidth Δλ of about 2-2.5% of its peak wavelength λ, so that a 1550 nm SLD would typically emit light having the FWHM bandwidth of about 35 nm, while such applications as low-coherence interferometers would benefit from a light source with a much broader spectrum, since their resolution is inversely proportional to the spectral bandwidth of the used light source.
A solution to this problem is proposed in U.S. Pat. No. 6,184,542 in the name of G. A. Alphonse, which discloses a multi-section GaAs based SLD shown in FIG. 1, which reproduces FIG. 4a of the '542 patent.
Similar to a conventional SLD, the device of Alphonse has an n type cladding layer 3 that is deposited on a substrate 2, which is followed by an active layer 10 and a p type cladding layer 5. The refractive index of the active layer 10 is greater than the refractive index of the two cladding layers 3, 5 to provide a waveguiding effect in the direction normal to the layers. A capping layer 6 is deposited on the p type cladding layer 5. After the capping layer 6 is deposited, photolithography and etching is performed to define the waveguide as a ridge 8 with channels 9 on the sides. The channels are patterned and the capping layer 6 and the cladding layer 5 are etched down to an etch stop layer, not shown, within the cladding layer 5. Thus, in the channels, a small portion of the cladding layer 5 overlies the active layer 10. A base electrical contact 1 is then deposited to overlie the surface of the substrate 2 opposite the n type cladding layer 3. The electrical contact 1 is an alloy including one or more of germanium, gold, and nickel. A dielectric is then deposited over the entire top surface of the structure. Using photolithography and etching, a stripe is opened over the ridge 8, and a metal, such as an alloy of titanium, platinum, and gold, is deposited therein on the capping layer 6 as a top electrical contact 7 in the stripe region to confine electrical current to the ridge region. The waveguide stripe of FIG. 1 is formed at an angle θ with respect to the facets a and b to avoid facet reflection, which would otherwise lead to the appearance of periodic undulations in the emission spectrum due to the etalon effect and, ultimately, to the spectrum narrowing due to lasing.
To broaden the SLD emission spectrum, the active layer 10 is formed of three neighboring emission layers 15, 20, 25 of differing material composition aligned along the length of the device as illustrated in FIG. 2 in cross-section, so that each material 15, 20, and 25 has a different bandgap and therefore different center emission wavelength, i.e. λ1, λ2, or λ3, respectively. It is proposed in the '542 patent that, if these center emission wavelength are selected so that respective spectra of the amplified spontaneous emission (ASE) from each of the emission layers 15, 20, and 25 overlap, passing of a suitable electrical current between the electrical contacts 1 and 7 would produce broad-band light output from the device having a substantially flat spectrum in the λ1 to λ3 range with a FWHM spectral width Δλ that is the sum of the FWHM spectral widths Δλi, i=1, 2, 3, of each individual ASE spectrum, Δλ=Δλ1+Δλ2+Δλ3.
However, we found that an SLD of the type described in the '542 patent tends to have an un-even, rather than flat, emission spectrum that is difficult to control and that is typically less spectrally broad than expected on the basis of the bandgap spread of the used material, e.g. less broad than combined radiation from three conventional SLDs with the differing center emission wavelength λ1, λ2, and λ3.
An object of the present invention is to overcome at least some of the deficiencies of the prior art by providing a multi-section semiconductor light emitting device having a plurality of light emission regions that are individually addressable electrically for emitting broad-band light with controllable spectral profile.